A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).
To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.
Low-k1 lithography may be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such process, the resolution formula may be expressed as CD=k1×λ/NA, where λ is the wavelength of radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the “critical dimension” (generally the smallest feature size printed, but in this case half-pitch) and k1 is an empirical resolution factor. In general, the smaller k1 the more difficult it becomes to reproduce the pattern on the substrate that resembles the shape and dimensions planned by a circuit designer in order to achieve particular electrical functionality and performance. To overcome these difficulties, sophisticated fine-tuning steps may be applied to the lithographic projection apparatus and/or design layout. These include, for example, but not limited to, optimization of NA, customized illumination schemes, use of phase shifting patterning devices, various optimization of the design layout such as optical proximity correction (OPC, sometimes also referred to as “optical and process correction”) in the design layout, or other methods generally defined as “resolution enhancement techniques” (RET). Alternatively, tight control loops for controlling a stability of the lithographic apparatus may be used to improve reproduction of the pattern at low k1.
In lithographic processes, it is desirable to make frequent measurements of the structures created, e.g., for process control and verification. Tools to make such measurement are typically called metrology apparatus MT. Different types of metrology apparatus MT for making such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology apparatus MT.
Recently, various forms of optical tools or scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a measurement signal from which a property of interest of the target can be determined.
Known inspection techniques employ radiation in the visible or ultraviolet waveband (e.g. greater than 200 nm). This limits the smallest features that can be measured, so that the technique can no longer measure the smallest features made in modern lithographic processes. To allow measurement of smaller structures, it has been proposed to use radiation of shorter wavelengths similar, for example, to the extreme ultraviolet (EUV) wavelengths used in EUV lithography. Such wavelengths may be in the range 1 to 100 nm, for example, or 1 to 125 nm. Part or all of this wavelength range may also be referred to as soft x-ray (SXR) wavelengths. Some authors may use SXR to refer to a narrower range of wavelengths, for example in the range 1-10 nm or 10-20 nm. For the purposes of the methods and apparatus disclosed herein, these terms SXR and EUV will be used without implying any hard distinction. Metrology using harder x-rays, for example in the range 0.1-1 nm may also be used.
For SXR metrology a source may be used that works on the principle of higher harmonic generation (HHG). A high power pulsed IR drive laser is focused in a gas-jet, in which a small fraction of the power is converted to shorter wavelengths. The generated wavelengths follow the odd harmonic orders: λ_generated=λ_drive/n where n is odd.
The source is subsequently demagnified in an optical illumination system and focused on a target on the wafer. The focus may underfill the target. Since the target is very small this is challenging both from a static point of view (spot needs to be small) and a dynamic point of view (spot needs to stand still). Small source positioning errors will lead to spot to target displacements and result in erroneous measurement results.
The size of SXR spots incident on detectors of a metrology apparatus is small, typically on the order of a number of detector pixels. This leads to tight requirements on the angular stability of the source: small beam pointing errors of the source result in changes in SXR spot positions on far field detectors. It may be desirable to have an angular stability as small as several μrad (micro-radians) at the source position.
Previous methods for stabilizing source positioning involved metrology on the IR drive laser. However, this is an indirect measurement and the source positioning and beam pointing of the SXR may well be subject to instabilities in the gas-jet (density variations, turbulence, plasma), resulting in fluctuations not captured by diagnostics on the IR.